Whenever gaseous fluids are moved significant distances through pipes, they may change character as less volatile or cooler fractions condense into the liquid phase. The liquid moves slower and gathers in low spots. The slugs of liquid are then forced along in spurts of sudden, erratic, and unpredictable movement that interfere with the efficient operation of the overall system. For example, natural gas may have entrained water vapor that condenses into liquid water that impedes the flow of gas vapor through the gas line.
The most common example of this problem is experienced in steam distribution lines. Eventually, the cooler walls of the pipe condense the steam into water. If the steam is being used in a chemical process, the water may degrade the desired reactions. A typical use for large steam distribution systems is where steam is injected into oil wells to enhance recovery. A central steam injection system usually supplies many wells and the steam is often conveyed through miles of pipe to reach the well heads. The efficiency of oil recovery depends on the quality and heat content of the steam when it is injected into the well. It is, therefore, necessary to be able to measure the relative amounts of steam vapor and liquid water passing through the pipe on a real time basis. But this measurement has been difficult to achieve using prior art techniques.
The general problem solved by this invention is the measurement of the relative amounts of vapor and liquid passing through a pipe. Since steam and water is the hardest mixture to measure, and also the most economically desirable, this description emphasizes steam and water mixtures. But it should be understood that all of the principles of the invention apply equally to any mixture of vapor and liquid.
Since vapor, or the gas phase, is lighter and less dense than liquid, if the overall density of the contents of a pipe can be determined, then the ratio of liquid to gas can be inferred therefrom. The more dense the contents, the more liquid phase must be present. One prior art method to sense density is to employ a nuclear densitometer. The amount of gamma radiation passing through the pipe from a radioactive source is measured. If the average density of the pipe contents rises, the gamma radiation is blocked and declines. But this approach is very expensive and the handling of a radioactive source is difficult and subject to much government regulation. Also, the source is not stable but continuously declines, requiring frequent recalibration and questionable long term accuracy. The present invention provides a more accurate, less expensive, and safer apparatus to measure the density of a vapor and liquid mixture flowing through a pipe.